More than one third of individuals with epilepsy are not seizure-free with currently available treatments. Causes of intractable epilepsy are manifold; yet, elevated and uncontrolled neuronal excitability and synchronicity in the brain are fundamental attributes. Therapies that exploit the brain's intrinsic mechanisms to control excitabil- ity and suppress synchronicity thus hold great promise, but critically depend on a better understanding of these processes. The contribution of the proposed research to this challenge will be to assess microRNA-induced silencing of transient inactivating A-type potassium currents as an inherent mechanism of the brain that con- trols neuronal excitability and could be a therapeutic target for epilepsy. A-type currents are crucial gate keep- ers of excitability and synchronicity in the brain, and defects in A-type current-mediating channel subunits in- crease the susceptibility to seizures and epilepsy. Manipulating these currents could thus be therapeutic in epi- lepsy, but there are currently no specific A-type channel-modulating drugs available. In the brain, A-type cur- rents are mainly mediated by the potassium channel Kv4.2 and its auxiliary subunits (Kv4.2 complex). Protein levels of the Kv4.2 complex are decreased in rodent models of epilepsy, suggesting that downregulation of the Kv4.2 complex is a pathological mechanism in epilepsy. If understood in detail, the mechanisms that lead to this downregulation of the Kv4.2 complex could thus serve as an alternative drug target to manipulate A-type currents. The central hypothesis of this project is that downregulation of Kv4.2 and A-type currents in epilepsy is caused by microRNA-mediated silencing of the Kv4.2 complex, which contributes to neuronal hyperexcitabil- ity and -synchronicity and could thus be a therapeutic target. This hypothesis is supported by strong pilot data showing that inhibition of a Kv4.2-targeting microRNA decreases severity of provoked status epilepticus in mice, reduces kainic acid-induced excitotoxicity in cultured wild type, but not Kv4.2 KO neurons and prevents kainic acid-induced downregulation of Kv4.2 in vitro. Three aims will be pursued. Aim 1 will examine if inhibition of Kv4.2-targeting microRNAs prevents Kv4.2 complex downregulation following seizures and reduces seizure frequency and severity in a mouse model of acquired epilepsy. Aim 2 will identify the most potent Kv4.2- targeting microRNAs by analyzing the mechanisms and quantitatively comparing the efficacy of candidate mi- croRNAs to regulate Kv4.2 complex expression and function. Aim 3 will test the physiological relevance of these findings by expanding the analyses to two mouse models of genetic intractable epilepsy, neuron-specific Pten deletion mice and Cntnap2 KO mice. In contrast to Cntnap2, Pten deletion leads to reduced Kv4.2, ena- bling assessment of increased microRNA-induced Kv4.2 silencing as pathological mechanism and therapeutic target in epilepsy with and without detectable Kv4.2 defects. The approach is innovative, because it will manipulate A-type current expression to modify function. The research is expected to advance knowledge about how the brain regulates excitability, which ultimately could lead to new strategies to treat intractable epilepsy.